Long Island’s water safety is complex because the entire region relies on a single, vulnerable source for its supply. Every drop of water used by residents and businesses in Nassau and Suffolk counties is drawn exclusively from underground reservoirs. This unique dependence means that protecting the source is an ongoing challenge, as decades of human activity have introduced pollutants that require advanced and costly treatment.
The Sole-Source Aquifer System
Long Island’s drinking water comes entirely from a deep, multi-layered aquifer system, which the U.S. Environmental Protection Agency (EPA) has designated a Sole Source Aquifer. This designation confirms that there is no alternative surface water supply, such as rivers or reservoirs, available to the population. The system is composed primarily of three major layers of sand and gravel: the Upper Glacial, the Magothy, and the Lloyd Aquifers.
The shallowest layer, the Upper Glacial Aquifer, is the most susceptible to contamination because of its proximity to the surface. It is the first layer that rainfall must pass through to recharge the system. The Magothy Aquifer, the largest and most heavily utilized, lies beneath the Upper Glacial and provides the majority of the region’s drinking water.
The deepest and oldest layer is the Lloyd Aquifer, which contains water that can be thousands of years old. The entire system functions like a massive underground sponge, collecting and storing precipitation that filters down through the sandy soil. The vulnerability of the Upper Glacial layer means that pollutants from the ground surface can easily percolate downward and migrate into the deeper, more heavily pumped Magothy layer.
Primary Contaminants Affecting Long Island Water
The primary water quality challenges on Long Island involve “emerging contaminants” and common agricultural runoff, both resulting from historical and ongoing surface activities. Two types of synthetic chemicals, Per- and Polyfluoroalkyl Substances (PFAS) and 1,4-Dioxane, have been detected widely in the groundwater. PFAS compounds, which include PFOA and PFOS, are often called “forever chemicals” because they resist breakdown in the environment, originating from sources like industrial sites and specialized firefighting foams.
The synthetic industrial solvent stabilizer 1,4-Dioxane, classified as a probable human carcinogen, is another significant concern. This chemical was historically linked to manufacturing processes but also appears in trace amounts as a byproduct in common household and personal care products like shampoos and detergents. Because it mixes easily with water and is highly stable, it has formed widespread contamination plumes across the aquifer system.
Nitrates represent a third major contaminant, primarily stemming from fertilizer runoff and the effluent from septic systems and cesspools across the island. Elevated nitrate levels are particularly concentrated in the shallow Upper Glacial Aquifer, which is more directly impacted by these surface sources. This contamination is a long-term problem that affects not only drinking water quality but also the health of the island’s surface waters and estuaries.
Utility Treatment and Regulatory Oversight
Long Island’s public water suppliers are required to treat water to meet stringent regulatory standards set by the New York State Department of Health (NY DOH). New York has adopted some of the strictest Maximum Contaminant Levels (MCLs) in the nation for emerging contaminants, including 10 parts per trillion (ppt) for PFOA and PFOS, and 1 part per billion (ppb) for 1,4-Dioxane. These limits are enforceable and force water districts to invest in advanced infrastructure to ensure compliance.
To meet these tight standards, water suppliers are increasingly deploying sophisticated treatment technologies. The approved method for removing 1,4-Dioxane is Advanced Oxidation Process (AOP), which typically involves adding an oxidant like hydrogen peroxide and exposing the water to ultraviolet light to destroy the chemical molecules. This AOP step is often paired with Granular Activated Carbon (GAC) filtration.
GAC, which uses large beds of activated carbon, is highly effective at removing PFAS compounds and the residual oxidant left over from the AOP process. These multi-stage treatment plants represent a substantial investment, with the estimated cost for full compliance across the island reaching into the billions of dollars.
Public water systems must regularly monitor for over 100 contaminants and publish their findings annually in a Consumer Confidence Report (CCR). The combination of state-mandated MCLs and the deployment of AOP/GAC systems ensures that the water delivered by public utilities is actively treated to remove these problematic contaminants.
Steps for Ensuring Water Quality at Home
For residents served by a public water system, the first step is to review the Annual Water Quality Report, also known as the Consumer Confidence Report, which is mailed out yearly. This document provides a table showing the detected levels of contaminants compared to the established MCLs. Understanding the report helps residents verify that their local water district is meeting the state’s stringent limits.
If a resident seeks an extra layer of protection, particularly for emerging contaminants, installing a point-of-use (POU) filtration system is an option. Effective home filtration against contaminants like PFOA, PFOS, and 1,4-Dioxane usually involves systems certified by independent organizations like NSF International. Consumers should look for systems certified to NSF/ANSI Standard 53 or 58, which cover the use of carbon filtration and reverse osmosis, respectively, for the reduction of specific health-related contaminants.
Reverse Osmosis (RO) systems are the most comprehensive POU option, capable of removing up to 99% of dissolved solids and many chemical contaminants, including PFAS. Carbon-based filters, such as those certified to reduce PFOA/PFOS, are also a viable option but must be replaced regularly to maintain effectiveness.
Residents with private wells, who do not benefit from municipal monitoring and treatment, should have their water professionally tested. Testing should occur at least annually for bacteria and every few years for chemical contaminants, including nitrates and volatile organic compounds.